This invention relates to a pneumatic servomotor used for assisting in the braking of a motor vehicle.
More particularly, the present invention relates to a pneumatic servomotor for an assisted braking of a motor vehicle, of the type comprising a rigid casing, in which a transverse partition wall is movable, thus defining in an airtight manner a front chamber under a first engine negative pressure, and a rear chamber under a second pressure, varying between the engine negative pressure and the atmospheric pressure; of the type including a moving piston, integral with the moving partition wall, and a control rod, moving inside the piston, in a selective manner as a function of an axial input force, exerted in the forward direction against a return force applied to the rod by a return spring; of the type according to which the control rod is biased towards either an intermediate actuation position or an end actuation position, resulting from the applying of the input force at a determined high speed; of the type comprising a plunger, arranged at the front part of the control rod inside the piston, and a three-way valve, including at least an annular seat borne by a rear section of the plunger, and capable of varying the second pressure, prevailing within the rear chamber, particularly by connecting the front chamber with the rear chamber when the control rod is in the rest position, or by gradually connecting the rear chamber with the atmospheric pressure when the control rod is actuated; of the type in which, in the end actuation position of the control rod, a finger, which is slidingly fitted on the front end of the plunger, is biased by the plunger into contact with a reaction disk, integral with the rear face of the moving piston, so as to transmit the reaction force of the moving piston to the plunger and to the control rod; and of the type comprising a unidirectional clutch device, which includes a coaxial sleeve, sliding on the plunger, and a substantially ring-shaped key, arranged with a given clearance around the sleeve and capable of being driven by the moving piston when the input force is applied at said determined speed, so as to rotate on a generally transverse axis in order to cooperate with the periphery of the sleeve and lock it in an end front axial position, in which it locks the finger, independently of the plunger and of the control rod.
In a well-known manner, such a design is most suitable as regards safety, in the case of an emergency braking situation.
As a matter of fact, a conventional servomotor comprises neither an independent finger nor a unidirectional clutch device for the finger. A finger-forming end section of the plunger is likely to bias directly the reaction disk, integral with the rear face of the moving piston.
In a full-braking situation, in which case a maximum braking force is applied on the control rod, the actuation of the control rod causes the finger-forming plunger to be actuated, which fact results in the maximum opening of the three-way valve and, therefore, the rear chamber is subjected to the atmospheric pressure. Thus, the moving partition wall travels forward and the end of the plunger contacts the reaction disk, integral with the rear face of the moving piston.
Therefore, the force, which is applied onto the moving piston when the control rod reaches the end of its stroke, results from the assisting force, arising from the pressure difference between each side of the moving partition wall, and from the force exerted by the finger-forming plunger onto said moving piston. Besides, the driver feels the braking reaction force, which is transmitted from the moving piston to the plunger, through the reaction disk.
As a matter of fact, it has been established that quite a number of drivers, when confronted with an emergency braking situation, underestimated the risks actually incurred and, after having jammed the brakes on, would somewhat release the braking force at the very time when a braking force should have been maintained in order to avoid an accident.
In the case of a full-braking situation, accompanied by the swift travel of the control rod, the plunger may touch the reaction disk and therefore give the driver the feeling of a maximum braking action even before the pressure difference between the front and rear chambers actually reaches its maximum value, which may lead the driver to release the braking force even though it should be maintained so as to profit by the maximum braking force.
A servomotor, like that of the above-described type, makes it possible to eliminate such a disadvantage, in that the plunger is locked into contact with the reaction disk, by means of the sleeve, which results in a maximum force being maintained on the rear face of the moving piston, even though the driver may have released the braking force in part.
Yet, the servomotor of the previously described type has the disadvantage, unlike a servomotor without a unidirectional clutch device, of requiring, as it is manufactured, the separate machining of a sleeve and of a finger, since the axial fixing of the sleeve in position is ensured but in a rather inaccurate manner.
As a matter of fact, since the key locks the sleeve simply by wedging, that is through a very small contact area, not to say an almost pinpoint one, between an edge of a circular opening made in the key and the periphery of the sleeve, the sleeve is likely to slip when the key comes into contact with it.
If the key is not correctly positioned on the sleeve, in a full-braking operation, one incurs the risk that, if the finger is made in one piece with the sleeve, the sleeve may be locked in a position in which its finger would not bias the reaction disk, integral with the moving piston.
In order to cope with said difficulty, the present invention provides a one-piece design for the sleeve and the finger, associated with reliable means for an accurate fixing of the sleeve in position.
Therefore, it is the object of the present invention to provide a servomotor of the above-described type, characterised in that the finger is axially integral with the sleeve, and in that means are provided for an axial indexing of the locking position of the sleeve.
According to other features of this invention:
the finger and the sleeve are manufactured as a single piece from the same material;
the sleeve comprises a finger-forming front free end section, which is slidably fitted within a complementary front bore of the piston;
the sleeve has an intermediate cylindrical section having a greater diameter than the front free end section of the sleeve and which is slidably fitted within a complementary rear bore of the piston;
resilient means are mounted between a radial shoulder on the piston, defining the front and rear bores, and a front shoulder on the sleeve, which defines the front end section and the intermediate section, so as to return the sleeve axially rearwards in abutment against the plunger;
the resilient return means comprise a compression helical spring, arranged on the front end cylindrical section of the sleeve;
the plunger is slidably fitted inside a hole made in the sleeve, a bottom front face of which provides the axial support for the plunger in the end actuation position of the control rod;
the sleeve includes at least one locking transverse face, axially facing rearwards so as to constitute a stop for a peg provided on the key and extending radially towards the sleeve, for an axial indexing of the end locking position of the sleeve;
the sleeve has a cylindrical shape with a substantially constant diameter and it comprises a radial groove exhibiting a substantially truncated-cone-shaped profile, a shoulder-forming front transverse face constituting the locking transverse face;
the key has, in an axial sectional view, the shape of a tee, the vertical branch of which is substantially radially directed and traversed by the sleeve, whereas its horizontal branch bears the peg, which protrudes from the front part and has the shape of a concave truncated-cone-shaped angular sector, which is complementary to the truncated-cone-shaped profile of the groove in the sleeve, said branch being shaped, at both ends, into an arc of a cylinder so as to be received, without any axial clearance, between two walls of a cavity traversing the piston, perpendicularly to its axis, so as to allow but a rocking motion of the key inside the cavity, the key being further resiliently biased against the rear wall of the cavity.